Perhaps you already have the problem that the firefox needs to much RAM.
Sometimes it can grow up to 800 MB and more. The reason for this problem could be one of the addins or even the system which caches internet sites in order to load them faster.
And because of that it could happen that firefox slows down the whole system.

But with this workaround you can solve the problem

Enter about:config in your address bar

Perform a right click in the textwindow and select New >> Boolean

Enter config.trim_on_minimize in the textbox

In the following dialog select true

Now you should see a new entry with the name config.trim_on_minimize

Restart Firefox

When you minimize Firefox you can see in the Taskmanager how it trims the memory